Back­ground. ­Some ­reports ­have ­observed the ­response of cere­bral ­blood ­flow to ­PaCO2 ­during hypo­thermic car­di­o­pul­mo­nary ­bypass. We ­studied the ­effect of ­PaCO2 on the cere­bral cir­cu­la­tion ­during hypo­thermic selec­tive cere­bral per­fu­sion.­Methods. ­Between ­June 1992 and Jan­uary 1998, 35 ­patients under­went ­aortic ­arch ­grafting ­using hypo­thermic selec­tive cere­bral per­fu­sion (20ºC). In the ear­lier ­four ­patient (­Group 1), ­carbon ­dioxide gas was not ­added. In the ­latter 31 ­patient (­Group 2), ­carbon ­dioxide gas was ­added to the cere­bral per­fu­sion. The hemo­dy­namics and ­rates of ­change in cere­bral ­oxygen sat­u­ra­tion ­were eval­u­ated.­Results. In ­Group 1, the ­index of cere­bral arte­rial resis­tance was 9.2±2.2 at the ­start of selec­tive cere­bral per­fu­sion and ­increased to 15.7±0.1 at the re-­warming ­stage (p<0.05), and ­there was a sig­nif­i­cant ­decrease in cere­bral ­oxygen sat­u­ra­tion at the re-­warming ­stage (p<0.001). In ­Group 2, the ­index of cere­bral arte­rial resis­tance was 4.7±1.7 at the ­start of selec­tive cere­bral per­fu­sion and 4.3±1.5 at the re-­warming ­stage, a non-sig­nif­i­cant ­change. The ­change in cere­bral ­oxygen sat­u­ra­tion was ­also non-sig­nif­i­cant ­between the ­start of selec­tive cere­bral per­fu­sion and the re-­warming ­stage. ­Among the neu­ro­log­ical out­comes, ­there was ­only one ­small cere­bral infarc­tion in ­Group 2; how­ever, no ­delayed con­scious ­recovery was ­observed.Con­clu­sions. The addi­tion of CO2 to cere­bral per­fu­sion was a ­factor in inhib­iting the ­increase in the cere­bral vas­cular resis­tance at the re-­warming ­stage.